Yes you can. In fact you can water anyone's flowers. You can click on their avatar and select "Visit xxx's farm" to get to their farm.
![](https://r1.slashkey.com/images/gameguide/flowers/buddy icon.png)
To get to a buddys farm. Click on the yellow tool box then click on the Buddy icon.
![](https://r1.slashkey.com/images/gameguide/flowers/green man buddy.PNG)
Then click on the green man next to thier avatar name which will take you to their farm.